1. Field of the Invention
This invention relates generally to rotary drag bits for drilling subterranean formations, and more specifically to polycrystalline diamond compact (PDC) cutting structures for use with such rotary drag bits.
2. State of the Art
Fixed-cutter rotary drag bits have been employed in subterranean drilling for many decades, and various sizes, shapes and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements. Polycrystalline diamond compact (PDC) cutting elements, comprised of a planar diamond table formed under high temperature high pressure conditions onto a substrate typically of cemented tungsten carbide (WC), were introduced into the market about twenty years ago. PDC cutting elements, with their large diamond tables (usually of circular, semi-circular or tombstone shape), have provided drag bit designers with a wide variety of potential cutter deployments and orientations, crown configurations, nozzle placements and other design alternatives not previously possible with the smaller natural diamond and polyhedral, unbacked synthetic diamonds traditionally employed in drag bits. The planar PDC cutting elements have, with various bit designs, achieved outstanding advances in drilling efficiency and rate of penetration (ROP) when employed in soft to medium hardness formations, and the larger cutter dimensions and attendant greater protrusion or extension above the bit crown have afforded the opportunity for greatly improved bit hydraulics for cutter lubrication and cooling and formation debris removal. The same type and magnitude of advances in drag bit design for drilling rock of medium to high compressive strength have, unfortunately, not been realized.
State-of-the-art planar, substrate-supported PDC cutting elements have demonstrated a notable susceptibility to spalling and fracture of the PDC diamond layer or table when subjected to the severe downhole environment attendant to drilling rock formations of moderate to high compressive strength, on the order of nine to twelve kpsi and above, unconfined. Engagement of such formations by the PDC cutting elements occurs under high weight on bit (WOB) required to drill such formations and high impact loads from torque oscillations. These conditions are aggravated by the periodic high loading and unloading of the cutting elements as the bit impacts against the unforgiving surface of the formation due to drill string flex, bounce and oscillation, bit whirl and wobble, and varying WOB. High compressive strength rock, or softer formations containing stringers of a different, higher compressive strength, thus generally produces severe damage to, if not catastrophic failure of, the PDC diamond tables. Furthermore, bits are subjected to severe vibration and shock loads induced by movement during drilling between rock of different compressive strengths, for example, when the bit abruptly encounters a moderately hard strata after drilling through soft rock.
Severe damage to even a single cutter on a PDC cutting element-laden bit crown can drastically reduce efficiency of the bit. If there is more than one cutter at the radial location of a failed cutter, failure of one may soon cause the others to be overstressed and to fail in a "domino" effect. As even relatively minor damage may quickly accelerate the degradation of the PDC cutting elements, drilling operators as a whole have lacked confidence in PDC cutting element drag bits for hard and stringer-laden formations.
It has been recognized in the art that the sharp, typically 90.degree. edge of an unworn, conventional PDC cutting element is usually susceptible to damage during its initial engagement with a hard formation, particularly if that engagement includes even a relatively minor impact. It has also been recognized that pre-beveling or pre-chamfering of the PDC diamond table cutting edge provides some degree of protection against cutter damage during initial engagement with the formation, the PDC cutting elements being demonstrably less susceptible to damage after a wear flat has begun to form on the diamond table and substrate.
U.S. Pat. Nos. Re 32,036, 4,109,737, 4,987,800, and 5,016,718 disclose and illustrate bevelled or chamfered PDC cutting elements as well as alternative modifications such as rounded (radiused) edges and perforated edges which fracture into a chamfer-like configuration. Co-pending U.S. patent application Ser. No. 893,704, filed Jun. 5, 1992, assigned to the assignee of the present application and incorporated herein by this reference, discloses and illustrates a multiple-chamfer PDC diamond table edge configuration which, under some conditions exhibits even greater resistance to impact-induced cutter damage.
However, even with the PDC cutting element edge configuration modifications recently employed in the art, cutter damage remains an all-too-frequent occurrence when drilling formations of moderate to high compressive strengths and stringer-laden formations. As a result, PDC cutting element drag bits are still employed less frequently than might be desired in drilling such formations in light of their aforementioned advantages due to the continued lack of confidence in their durability.
It would be desirable to provide a PDC cutting element with better protection against damage during the first part of a run, before the protective wear flat forms, and to maintain the pristine cutting edge in its original state until useful engagement with the formation is commenced. By prohibiting or significantly reducing initiation and propagation of diamond table fracture when the bit gets to the bottom of the borehole, the new, sharp, undamaged cutting edges can usefully engage the formation and develop protective wear flats which will inhibit damage during the remainder of the run. Thus, cutter life would be enhanced and prolonged.